The technical field of this invention is that of nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using eddy-current sensors. Characterization of bulk material condition includes (1) measurement of changes in material state caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from shot peening, roll burnishing, thermal-spray coating, or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, and coating condition. Each of these also includes detection of electromagnetic property changes associated with single or multiple cracks. Spatially periodic field eddy-current sensors have been used to measure foil thickness, characterize coatings, and measure porosity, as well as to measure property profiles as a function of depth into a part, as disclosed in U.S. Pat. Nos. 5,015,951 and 5,453,689.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field at the same frequency, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
One of the difficulties encountered when performing material property measurements with traditional spatially periodic field and other eddy current sensors is the limited sensitivity to flaws or defects, such as cracks, voids, inclusion, and corrosion, hidden behind metal layers or deep within the test material. A limiting factor for these measurements is often the frequency range of operation for the sensing device as it affects both the depth of penetration of the magnetic field into the test material and the detectable signal level of the device. The depth of penetration of the magnetic field into a material is determined by the geometry of the drive winding and the skin depth for the magnetic field in the material. The geometry affects the dominant spatial wavelength for the decay of the field into the material. For measurement sensitivity to a hidden flaw, both the dominant spatial wavelength and the skin depth need to be comparable to, or larger than, the thickness of material between the flaw and the sensor. Since the skin depth varies inversely with the square root of the frequency, sensitivity to deep flaws requires low excitation frequencies. The excitation frequency also affects the output signal level, which is the induced voltage from a secondary coil or winding for a traditional eddy current sensor. This voltage is proportional to the rate of change of the magnetic flux through the coil, and hence the excitation frequency. Since the induced voltage decreases with frequency, the lowest detectable signal level determines the lowest frequency of operation for the sensor, which may not be low enough for the detection of a flaw, according to the skin depth.
In addition, new materials, manufacturing processes and structural designs, as well as new damage mechanisms, pose continual challenges to the state-of-the-art in non-destructive evaluation. In particular, thick sections and multi-layered structures create difficult to inspect areas in which corrosion or other damage can propagate undetected. Although X-ray and ultrasonics have become common for inspection of thick structures and components (lapjoints, friction stir welds, turbine engine disks, and structural castings) for defects and geometric features, these techniques are limited in their sensitivity and image resolution. More importantly, they provide little, if any, information on absolute material properties. There is a need for lower cost, higher speed, wide area scanning capabilities not only to image defects, hidden corrosion, and geometric features, but also to provide images of metallurgical properties and residual stresses (e.g., for ferrous alloys magnetic permeability varies directly with applied and residual stress).
Giant magnetoresistive (GMR) and magnetoresistive sensing elements have been used to address this issue. Goldfine et al., described the use of arrays of magnetoresistive sensors with meandering drive windings in U.S. Pat. No. 5,793,206 as an alternative to inductive coils. Wincheski et. al. at NASA (Wincheski, 2001) and Raymond Rempt of Boeing (Rempt, 2001) have used single sensing elements and arrays of GMR or magnetoresistive sensing elements to detect subsurface cracks or corrosion.
A GMR sensor offers substantial new capabilities at a very reasonable cost compared to competing technologies, such as SQUIDs. GMRs take advantage of the large magnetoresistive effect exhibited by certain metallic magnetic superlattices. Whereas normal magnetoresistive materials exhibit maximum changes in resistance on the order of 5% when exposed to magnetic fields, GMRs exhibit resistance changes of 20% or more. Giant magnetoresistance was first observed in Fe/Cr magnetic superlattices, where a drop of as much as 45% of the resistivity was measured at liquid helium temperature. At room temperature, the magnitude of the effect was reduced to about 12%. Other material systems have been tested since then, with the Co/Cu magnetic superlattice emerging as the system of choice in the development of practical sensors. It exhibits resistivity drops of up to 55% at liquid helium temperature and 40% at room temperature.
A quantitative physical model of the giant magnetoresistive effect, developed by R. Q. Hood and L. M. Falicov, concludes that a large difference in interface scattering for the different spins is needed to explain the observed large GMR values. The magnetic superlattices have alternating layers of nonferromagnetic and ferromagnetic metals. The thickness of the nonferromagnetic layers is chosen such that in the absence of applied external magnetic field, the moments of consecutive ferromagnetic layers align antiparallel to each other. This antiferromagnetic coupling between these layers has been ascribed to indirect exchange interactions through the nonferromagnetic layers. The presence of an external field acts to align the moments of the ferromagnetic layers, resulting in reduction of the electric resistivity.
The sensitivity of GMRs to magnetic field strength and direction, as opposed to the rate of change of magnetic field strength, suggests the feasibility of a deep penetration eddy current type sensor. Typical eddy current devices lose sensitivity at lower frequencies. In order to achieve deep penetration of eddy currents, however, the excitation frequency must be decreased in order to increase the skin depth. The lack of sensitivity of simple inductive coils at low frequencies limits the depth of sensitivity of typical eddy current sensors in aluminum, for example, to a few millimeters. Replacing inductive coils or sense elements with GMR sensing elements has the potential to increase the depth of sensitivity. The term depth of sensitivity, not penetration, is used because the sensing elements do not affect the magnetic field depth of penetration provided by the drive windings.
Some progress has been made in adapting GMR sensors to non-destructive testing applications. Wincheski and Namkung have integrated a GMR with a self nulling probe driver coil to produce the Very Low Frequency (VLF) Self Nulling Probe. They have operated this device at excitation frequencies down to 135 Hz and have used it to detect an EDM notch at a depth of up to 10 mm (Wincheski, 2001). However, the need still exists to improve measurement reliability and robustness with GMR and magnetoresistive sensing element for eddy current and also for DC measurements with current driven drive windings.